Hologon disks, i.e. disks bearing one or more holographically formed diffraction gratings as facets, have been used as rotating light deflectors in various commercial systems, e.g., shaft position detection/control systems, bar code reader systems and non-impact printing systems. The hologon disks have been embodied in a variety of rotational configurations. For example, U.S. Pat. No. 4,610,500 describes a construction having attachment means extending through the center of the disk to couple it to a rotating shaft. U.S. Pat. No. 5,016,955 discloses a system wherein the disk is engaged around its periphery by an element forming the rotor of a pancake motor. U.S. Pat. No. 4,528,448 suggests constructions where the disk is attached to a rotatable shaft by "some form of adhesive" or even formed on the end of such a shaft.
To a large extent the application for the beam deflector and performance requirements for the output beam determine the constructional forms of hologon scanner to be used. For example, the desired scan rate, the scan spot intensity, the locational accuracy and the intensity uniformity are important considerations. Single facet disks such as described in U.S. Pat. No. 5,046,794 offer certain advantages, e.g.: requiring less precise substrate construction and being susceptible to rim support (without internal stress concentrations caused by a center hole, and thus capable of high rotational speeds). Center-rotated hologon disks that have a plurality of discrete deflection facets disposed around the periphery offer the advantages (over the single facet constructions) of a significantly increased number of scans per revolution and of being more easily embodied to: (i) operate in a transmissive mode using the first order diffraction beam, and (ii) facilitate air cooling. However, the multifacet disks present constructional challenges with respect to substrate and coating thickness uniformity, substrate flexure (due to centrifugal forces and thermal gradients) and substrate cracking due to internal stresses at high speed rotation.